1. Field of the Invention
The present invention relates to an image smoothing apparatus for printers, such as laser printers, light emitting diode (LED) printers or the like, which form dot images in a matrix array, and more particularly to an apparatus for smoothing images of curves or diagonals included in the dot images to obtain high quality images.
2. Description of the Prior Art
The prior art discloses a laser printer for forming a picture element or "pixel" pattern on a photosensitive drum along a coaxial scanning line in a matrix by repeatedly scanning laser beams with modulated video data sent serially from an image controller.
In another prior art technique, a dot printer (such as an LED printer or a thermal printer) forms pixel patterns in a matrix array on a recording member or on a photosensitive drum (which moves in a direction relatively vertical to the scanning line) by forming pixels arrayed in a line along the scanning line, and by being controlled to emit the full line simultaneously or successively, block by block.
In either printer, the pixels are arrayed in an n.times.m matrix to form characters or pictures. Letters consisting of curves or diagonals, such as "0" or "V", are formed with stairstep edges or boldfaced crossings having a plurality of pixels close to each other, as in the letter "X". This damages the quality of the picture.
To overcome defects in laser printers which form pixel patterns by scanning modulated laser beams along the scanning line, attempts at smoothing the stairsteps have been disclosed in which a smaller dot size is formed by reducing the output energy for the pixels at the diagonal edges (Japanese Laid Open Pat. Appln. 60-139072). Another technique narrowly modulates the width of the output beam pulse (the video clock) and prints the small dots within the standard dot area.
In the prior art techniques described above, however, it is difficult to treat for smoothing those stairsteps which correspond to the variational states of curves or diagonals, because such techniques are limited to modifying the pixel diameter, shifting the small pixels within the range of the video clock for the standard pixel.
A third prior art technique, therefore, as shown in Japanese Laid Open Pat. Appln. 60-251761, discloses a smoothing method of replacing a white dot with a smaller black dot, or with a coarse energy density pixel next to the specific pixel in the scanning or horizontal direction. This technique provides a method for simply adding a smaller dot next to a specific pixel. It does not improve the image quality of thin curved lines or diagonal lines, because it makes the thin line boldfaced. Nor does it offer an effective smoothing method for a slightly tilting line, because it simply adds a dot horizontally.
A technical means to remedy the defects above is provided in U.S. Pat. No. 4,847,641, wherein a small dot is not only added adjacent to the specific pixel, it is also formed by flattening a standard dot so that the addition of the small dot and the flattening of the standard pixel according to the small dot are subjected coordinately in the directions of the horizontal scanning line and of the vertical subscanning line too.
The method of the fourth prior art technique, as shown in FIG. 10, may be described as comprising: providing four compensation subcells 51a, . . . , 51d for producing dots narrower in the scanning direction, and four other compensation subcells 52a, . . . , 52d for producing dots narrower in the subscanning direction, serially loading bit data corresponding to image information in a first-in-first-out (FIFO) buffer 53, subsequently receiving the data in a plurality of line unit, extracting the bit data adjacent the specific bit out of a sampling window 54; transferring the bit data to a matching network 55; matching the bit data map in the window 54 with a plurality of templates 56 provided in the matching network 55; selecting, if matched, suitable compensation subcells with a subcell generator 57 for producing the dot(s) among the subcells 51b, 51c, 52c, 52d, and/or compensation subcells for adding the small dot(s) to the adjacent white pixel(s) among the subcells 51a, 51d, 52a, 52b; selecting, if not matched, the standard pixel to output serially the selected video data to a print engine driver 58 enabling the laser printer to operate as desired.
With the fourth prior art technique, therefore, a tilted vertical line, as shown in FIG. 9(a), is modified to produce a standard pixel 70 that is narrower in the horizontal direction along the scanning line, and to add a small dot next to the narrowed standard pixel opposite to the narrowed portion, as if the specific pixel had shifted horizontally. A horizontally inclined line, as shown in FIG. 9(B), is modified to produce a standard pixel 70 that is narrower in the vertical direction to the scanning line, and to add a small dot 71 in the rest space as if the specific pixel had shifted vertically. Thus, the fourth technique provides a desirable smoothing treatment to remedy the defects of the prior art previously stated.
The fourth prior art technique, however, still raises two issues.
The first issue lies in the configuration of the image elements. As stated above, the technique comprises a method of producing a narrower dot with a small dot added aside, as if the dot image was seemingly shifted to compensate for the stairstep. As shown in FIG. 9(A), the configuration of the image elements is formed with a constricted part 73 which may reveal a defect, particularly when the dot image is enlarged. This technique does not always form a closer approximation of the original analog continuous form.
Though the technique, further, tried to produce narrower subcells in both the vertical direction and the horizontal direction in coordination, it is difficult to make the dot narrower in both the vertical direction and the horizontal direction in coordination with an actual electrophotographic printer.
With a laser printer, for example, it is easy to make the dot narrower horizontally by means of beam modulation, which is based on the modification of the basic pulse width of video data. It is, however, difficult to make the dot narrower vertically, even if the beam energy intensity is reduced, and may result in merely reducing the diameter of the latent dot on a photosensitive drum.
On the other hand, in an LED printer it is easy to make a dot narrower vertically by means of controlling the time for emitting the LED array. It is more difficult, on the other hand, to modify the dot horizontally, because the LED elements are disposed at a fixed distance block after block.
The second issue of this prior art technique is the constitution of the electronic circuit.
To match the unique bit map of the sampling window with a number of pattern matching templates, the equipment must be provided with the standard pattern templates in a combination of each four sheets of templates for the dots adjacent to the central dot horizontally or vertically, and for the two dots of subcells which have to be selected for the adjacent dot. The number of templates, therefore, must be at least 256 (256=(4 sheets for horizontal dots.times.4 sheets for vertical dots).sup.2). Further, if the equipment is provided with a number of application templates for easier and more uniform smoothing treatment, the number of templates swells to an even greater extent. The large number of templates increases the volume of the memory and makes the comparing logic circuits very complicated, resulting in longer operation time which can not cope with the requirement for faster operation.
The prior art technique has revealed the second issue above, because the technique selects the subcells of two dots simultaneously for two axes, namely horizontal and vertical adjacent dots.
Further, the technique generates the sampling windows shifting consecutively the central dot piecewise, which results in matching for every dot in a multiple manner, which is apt to prolong the matching operation time.